Quarks are generally believed to never exist alone but only in color-neutral groups of two or three (and possibly five or more); all searches for free quarks since 1977 have yielded negative results. Quarks are differentiated from leptons, the other family of fermions, by color charge. In addition, leptons (such as the electron or the muon) have integral charge (+1, 0 or −1) while quarks have +2/3 or −1/3 charge (antiquarks have −2/3 or +1/3 charge).

1. Estimates of quark masses, being subject to considerable theoretical uncertainty, are controversial and still actively being investigated. There have been suggestions in literature that the u quark could be massless, but this is nearly ruled out by recent results.
2. Alternative names now deprecated. There was a short-lived attempt to name the second generation quarks "sideways" and "centre" to match the existing "up" and "down" but the more poetic names prevailed. Strangely the similar attempt to name the third generation quarks more poetically failed.
Ordinary matter such as protons and neutrons are composed of quarks of the up and down variety only. A proton contains two up quarks and one down quark, giving a total charge of +1. A neutron is made of two down quarks and one up quark, giving a total charge of zero. The other varieties of quarks can only be produced in particle accelerators, and decay quickly into the up and down quarks. (Electrons do not contain quarks, but are of a different type of particle called leptons).

One might wonder whether there are yet more families of quarks with even higher masses. Research at CERN has provided strong evidence that no such families exist. This experiment relied on accurate determination of the width in masses of the Z boson; by a subtle series of calculations, the numbers obtained could be shown to contradict the possibility that more families of quarks exist. See [1] for more information.

The number of families of quarks also affects the only other really high-energy situation we know of — the early Universe. The initial distribution of elements can be predicted using the Standard Model; any model with more heavy quarks would lead to a fraction of initial Helium-4 that is different from what is observed. Thus the number of quarks is confirmed by astronomical observations as well. See [1] for more information.

According to the theory of quantum chromodynamics (QCD), quarks
possess a property metaphorically called "color charge". Instead of just one charge type (with two signs, + and − in electromagnetism), color charge comes in 3 types. Quarks colors are called "red", "green", or "blue" to suggest the primary colors, while anti-quarks are anti-red or "cyan", anti-green or "magenta", and anti-blue or "yellow". Due to confinement (described below), only color-neutral or "white" particles can exist separately: particles possessing color must be part of a "white" composite. Particles composed of one red, one green and one blue quark are called baryons; the proton and the neutron are the most important examples. Particles composed of a quark and an anti-quark of the corresponding anti-color are called mesons.

Particles of different color charge are attracted and particles of like color charge are repelled by the strong nuclear force,
which is transferred by gluons, particles that themselves carry color charge (one color and one anti-color). Therefore, colors of quarks are not static, but are constantly changed by gluons, though the composite hadron always remains neutral. In addition to holding quarks together in mesons and baryons, a residual effect of the strong nuclear force holds the protons and neutrons together in the atomic nucleus.

Because the carriers of the strong force, the gluons, are themselves colored, the force between two quarks increases as the quarks are separated. Due to this mechanism, called confinement, quarks are never found free; they are always bound into color-neutral baryons or mesons. When we try to separate quarks, as happens in particle accelerator collisions, at some point it is more energetically favorable for a new quark/anti-quark pair to pop out of the vacuum than to allow the quarks to separate further. As a result of this, when quarks are produced in particle accelerators, instead of seeing the individual quarks in detectors, scientists see "jets" of many color-neutral particles (mesons and baryons), clustered together. This process is called hadronization or fragmentation, and is one of the least understood processes in particle physics.

The theory behind quarks was first suggested by physicistsMurray Gell-Mann and George Zweig, who found they could explain the properties of many particles by considering them to be composed of these elementary quarks. The name quark comes from "three quarks for Muster Mark", a phrase in James Joyce's Finnegans Wake. According to The American Heritageî Dictionary of the English Language: Fourth Edition, the word quark as used by Joyce is the standard English verb quark, meaning "to caw, croak." The reference to three quarks in Joyce's phrase caught Gell-Mann's attention as particularly suggestive of the particle's circumstances.